Protein K is an Escherichia coli outer membrane protein found associated with the K1 capsule (a capsule associated with pathogenic strains). We have cloned the structural gene for protein K, purified the protein, demonstrated that it functions as a pore-forming molecule and examined the genetic control of its synthesis. We have also shown that any one of four distinct porin proteins is required for biosynthesis of the E. coli K1 capsule and that capsule biosynthesis in a porin-negative strain leads to the death of the cell.